I like Di Grassi, he's probably the best driver we have from the "new generation" (and probably the last, since we only got crap stock cars now). He did a great GP2 season and wasn't in a top team (or was?).
There's Senna as well, but I don't think he's "ready", so to speak. I mean he has potential, but can you put a guy that has been racing for 4 years in F1?
PS: Piquet's problem might come from the fact he has always raced for his father. Renault was the first time, if I'm not mistaken, he's on his own. You can say his name put him where he is now, wich seems to be the opposite of Bruno Senna.